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an as yet unresolved competition between more heat available near the surface
from summer insolation, but stronger stratification inhibiting the upward mixing
ofthisheat.
1.2 The Southern Ocean
Sea ice extent in the Antarctic has not exhibited the marked decrease seen in the
north; however, the Southern Ocean sea-ice pack is largely seasonal to begin with,
and presents a whole set of interesting contrasts with the perennial pack of the
Arctic. The Southern Ocean is completely unbounded by land to the north, and in
generaltermsthewesterlywindsnearthewintericeedgetransportice(anditsfresh
water) northward, leaving the Antarctic IOBL much saltier than its Arctic coun-
terpart, with much less density contrast between it and the underlying warm deep
water(WDW).
AconsequenceofweakstratificationintheoceanaroundAntarctica,particularly
intheWeddellSea,ismoredirectcommunicationbetweenthesurfaceandthedeep
ocean. In the eastern part of the Weddell Gyre (near the Prime Meridian), even in
latewintertheicepackmaybelessthanhalfameterthick.Givenatmosphericcon-
ditions influenced by the Antarctic continent, this requires on average an upward
heat flux from the WDW of between 25 and 40Wm
−
2
, at least an order of mag-
nitude greater than in the Arctic (Parkinson and Washington 1979; Schlosser et al.
1990;Gordon1991;McPheeetal. 1999).
The lack of stratification also admits the possibility of deep reaching convec-
tion over the deep basin, with massive direct exchange of heat and other contam-
inants between the abyssal ocean and the atmosphere (one of the few places on
earth where this is possible). In the mid-1970s,shortly after the first availability of
satellite microwave imagery in the Southern Ocean, a large expanse of open wa-
ter (or low concentration sea ice) was observed that remained for several winter
seasons, well within the limits of the seasonal ice pack, and far from the continen-
tal shelves (Carsey 1980; Martinson et al. 1981). This persistent opening, called
the Weddell Polynya, suggested deep convection that brought enough heat to the
surfacetopreventiceformationallwinteroveranareaapproaching10%ofthenor-
malseasonalice packin the Weddell. Field observationsconfirmedthe presenceof
a homogeneous “chimney” extending from the surface to 4,000m (Gordon 1981)
and extensive cooling of WDW during the Weddell Polynya years (Foldvik et al.
1985;McPhee 2003). Gordon(1991) interpreted the Polynya as manifestation of a
“thermal mode” where sea ice formation is prevented by upward ocean heat flux
and convection is driven by cooling alone. The presence of sea ice and its interac-
tion with WDW through the IOBL, discussed in Section 8.3, are critical elements
in determining what conditions might again initiate return to the thermal mode in
the Weddell or elsewhere in the Southern Ocean. Were it to become widespread it
would almost certainly have climate impact by venting massive amounts of heat,
carbondioxideandothercontaminantsfromthe deepocean.Indeed,itis naturalto
